1. Field of the Invention
The present invention relates to a load control system having a plurality of control devices and operable to control the amount of power delivered to a plurality of electrical loads from an AC power source, and more particularly, to a novel communication protocol for allowing the control devices of the load control system to communicate with each other.
2. Description of the Related Art
Typical load control systems are operable to control the amount of power delivered to an electrical load, such as a lighting load or a motor load, from an alternating-current (AC) power source. A load control system generally comprises a plurality of control devices coupled to a communication link to allow for communication between the control devices. The control devices of a lighting control system include load control devices operable to control the amount of power delivered to the loads in response to digital messages received across the communication link, or in response to local inputs, such as user actuations of a button. Further, the control devices of a lighting control system often include one or more keypad controllers that transmit commands across the communication link in order to control the loads coupled to the load control devices. An example of a lighting control system is described in greater detail in commonly-assigned U.S. Pat. No. 6,803,728, issued Oct. 12, 2004, entitled SYSTEM FOR CONTROL OF DEVICES, which is incorporated herein by reference in its entirety.
To communicate with each other, the control devices of prior art lighting control systems have each included a unique device address. The device addresses facilitate the programming of the lighting control system, for example, such that a load control device is responsive to an actuation of a button of a keypad. In some prior art lighting control systems, the control devices each comprised a dual-inline package (DIP) switch for manually determining the device address of the control device. The process of setting a DIP switch in order to assign an address to a control device can be difficult and challenging. Accordingly, the control devices of the prior art lighting control systems were often assigned incorrectly, leading to communication errors. Therefore, some prior art lighting control systems have executed “soft addressing” procedures in which one of the control devices assigns the unique device addresses one-by-one to each of the control devices.
Many prior art lighting control systems use polling techniques to allow the control devices to communicate with each other. In order to execute the polling technique, one control device of the lighting control system must first establish itself as a “master” device. Then, the master device is operable to sequentially transmit poll messages to and receive responses from each of the other control devices in the control system. The response to the poll message may comprise an event to report (e.g., the actuation of a button on a keypad or another high-priority event) or simply a status update message.
Therefore, there exists a need for a robust communication protocol that uses a polling technique and a low enough baud rate to provide for a free-wiring scheme, while still allowing special events to be executed in a timely manner.